1. Field of the Invention
The present invention relates to assemblies for connecting circuitry. More particularly, the invention relates to mounting assemblies and methods for providing electrical connection between electronic circuitry.
2. Brief Description of Related Technology
In recent years, the popularity of small-sized electronic appliances, such as camera-integrated video tape recorders and portable telephone sets, has made size reduction of large-scale integration desirable. As a result, chip size or chip scale packages are being used to reduce the size of packages substantially to that of bare chips. Such chip scale packages include a semiconductor chip mounted on a carrier substrate, which improves the characteristics of the electronic device while retaining many of the operating features, thus serving to protect semiconductor bare chips and facilitate testing thereof.
Upside down integrated circuits, commonly referred to as “flip-chips”, are now gaining popularity as well. Flip-chips are manufactured using solder bump technology, in which solder bumps are deposited on solder-wettable metal terminations on a die or chip and a matching pattern of solder-wettable terminations on the substrate. With flip-chips, the solder bumps are placed on the integrated circuit terminals while the chip is in wafer form, and then, after singulation, a chip is flipped and aligned to the circuit board substrate. A fluxing agent is applied and the solder bumps are re-flowed by heating to establish bonding between the chip and the substrate, with all the joints being made simultaneously by melting the solder.
When the resulting circuit board assembly is exposed to thermal cycling, the reliability of the solder connection between the circuit board and the chip often becomes suspect. Commonly, after a chip is mounted on a circuit board, the space between the chip and the circuit board is filled with a sealing resin (often referred to as underfill sealing) in order to reinforce against stresses caused by thermal cycling. Such underfill encapsulation has gained considerable acceptance in the electronics industry, with epoxy-based resin materials being most commonly used in such applications. Moreover, the expansion coefficients of the underfill sealing can be adjusted, for example, by the addition of low thermal-expansion fillers, such as glass or ceramics, thus reducing the level of thermal stress that develops between the substrate and the underfill sealing. The underfill sealing thus provides structural reinforcement, which delocalizes the thermal expansion stress, thereby improving heat shock properties and enhancing the reliability of the structure.
Also, the underfill material helps adhere the chip to the substrate. As such, the underfill material should exhibit high cohesive strength to the die and the circuit board surface, and retain significant strength within the environment encountered by the electronic device, for example, during heat-up and cool-down cycles associated with on/off powering of the electronics, as well as climatic changes in temperature and humidity.
Application of underfill sealing typically involves dispensing the underfill material onto one or more edges of the flip-chip assembly after it has been assembled and the solder bumps affixed to the substrate. Capillary action draws the underfill material through the gap between the chip and the substrate. Such underfill techniques are time consuming, and complete filling of the underfill sealing between the chip and the substrate can be difficult to achieve, thus reducing the protection level afforded through the underfill sealing.
In an attempt to overcome these issues and to eliminate processing steps, underfill sealants incorporating fluxing agents for bonding of the solder bumps have been proposed. For example, U.S. Pat. Nos. 5,985,043 and 5,985,486 disclose polymerizable fluxing agents, which act as an adhesive to bond the chip to the substrate. Such polymerizable fluxing agents are based on polycarboxylic acids having olefinic linkages, compositions of that are curable upon exposure to heat. The thinking here is that the underfill sealant incorporating such polymerizable fluxing agents can be applied to the chip during the wafer stage of chip manufacture, often referred to as wafer-applied fluxing underfill, in which a plurality of chips are manufactured in one piece and later cut into individual chips. By pre-applying onto the wafer the fluxing agent/underfill sealant combination, the chip should only need to be placed on the substrate, with solder re-flow and underfill curing occurring to affix the chip thereto. In practice, however, including the fluxing agent and underfill sealant in a single composition tends to compromise adhesion and mechanical strength of the underfill sealant.
Further, in some applications, the desirability of component removal may be of a concern, for example, when chip failure occurs. To avoid destroying or scrapping the entire assembly, it has been proposed to include reworkable adhesive materials in the assembly process. Such reworkable adhesive materials may be used as the underfill encapsulant adhesive, thereby providing reworkability to enable removal and replacement of one or more defective die. Such reworkable underfill materials typically involve thermally cleavable epoxy-based polymers, which will decompose rapidly when exposed to a high enough temperature. For example, U.S. Pat. No. 5,948,922 (Ober) and U.S. Pat. No. 5,973,033 (Ober), each refer to a certain class of compounds and compositions based on such compounds which, when cured, provide decomposable compositions capable of being reworked.
U.S. Pat. No. 5,512,613 (Afzali-Ardakani), U.S. Pat. No. 5,560,934 (Afzali-Ardakani) and U.S. Pat. No. 5,932,682 (Buchwalter), each refer to a reworkable thermoset composition based on a diepoxide component in which the organic linking moiety connecting the two epoxy groups of the diepoxide includes an acid cleavable acyclic acetal group. With such acid cleavable acyclic acetal groups forming the basis of the reworkable composition, a cured thermoset need only be introduced to an acidic environment in order to achieve softening and a loss of much of its adhesiveness.
U.S. Pat. No. 5,872,158 (Kuczynski) refers to thermosetting compositions capable of curing upon exposure to actinic radiation, which are based on acetal diacrylates, and reaction products of which are reported to be soluble in dilute acid.
U.S. Pat. No. 5,760,337 (Iyer) refers to thermally reworkable crosslinked resins to fill the gap created between a semiconductor device and a substrate to which it is attached. These resins are produced by reacting a dienophile (with a functionality greater than 1) with a 2,5-dialkyl substituted furan-containing polymer.
International Patent Publication No. PCT/US98/00858 refers to a thermosetting resin composition capable of sealing underfilling between a semiconductor device including a semiconductor chip mounted on a carrier substrate and a circuit board to which said semiconductor device is electrically connected. The composition includes about 100 parts by weight of an epoxy resin, about 3 to about 60 parts by weight of a curing agent, and about 1 to about 90 parts by weight of a plasticizer. Here, the area around the cured thermoset is to be heated at a temperature of about 190° C. to about 260° C. for a period of time ranging from about 10 seconds to about 1 minute in order to achieve softening and a loss of much of its adhesiveness.
The additional chemistry built into such reworkable polymers to provide the ability to controllably degrade under appropriate conditions, however, oftentimes detracts from the overall effectiveness of the underfill sealing in terms of strength, adhesion, and moisture resistance.
U.S. Pat. No. 6,121,689 discloses a semiconductor flip-chip package, which includes a polymerizable fluxing agent. FIGS. 1 and 2 herein depict flip-chip structures as set forth in the '689 patent. As is apparent, the flip-chip structure of FIG. 1 includes a chip 10 having solder bumps 14 pre-assembled on contact pads 24 on bottom surface 16 of chip 10 for electrical connection with solder pads 12 of a substrate 20 through the use of an encapsulant 22. In further embodiments, a fluxing adhesive may be used to adhere the chip 10 to the substrate 20. Moreover, as shown in FIG. 2, the structure may also include a multi-layer encapsulant material 36, including attachment and stress distribution layers 38 and 40, and thermoplastic reworkability layer 42. This thermoplastic reworkability layer is generally a meltable polymer such as a polyimide-siloxane copolymer. As shown in FIG. 2, flux adhesive 34 may be provided between the chip 10 and the substrate 20 for attachment of the chip 10 to the substrate 20. Adhesion and mechanical strength of the underfill sealant may be compromised due to the incorporation of the fluxing agent and the adhesive in a single composition. Also, when an integrated circuit chip includes an encapsulant having a fluxing adhesive incorporated therein, the fluxing adhesive may adversely affect the encapsulant material, thereby reducing the shelf stability or pot-life. Also, the use of a thermoplastic material as the reworkability layer provides the assembly with limited rework properties.
Notwithstanding the state of the art, it would be desirable for an integrated circuit chip having an underfill sealant material which provides excellent adherence and thermal shock properties while allowing the substrate with which it is to be used to be readily processed without compromising the physical properties of the materials or the assembly.